EGL¶
The current version of EGL in Mesa implements EGL 1.5. More information about EGL can be found at https://www.khronos.org/egl/.
The Mesa’s implementation of EGL uses a driver architecture. The main
library (libEGL
) is window system neutral. It provides the EGL API
entry points and helper functions for use by the drivers. Drivers are
dynamically loaded by the main library and most of the EGL API calls are
directly dispatched to the drivers.
The driver in use decides the window system to support.
Build EGL¶
Configure your build with the desired client APIs and enable the driver for your hardware. For example:
$ meson configure \ -D egl=enabled \ -D gles1=enabled \ -D gles2=enabled \ -D gallium-drivers=...
The main EGL library and OpenGL are enabled by default. The two
gles*
options after enable OpenGL ES 1.x and 2.x+. The last option enables the listed Gallium drivers.Build and install Mesa as usual.
In the given example, it will build and install libEGL
, libGL
,
libGLESv1_CM
, libGLESv2
, and one or more EGL drivers.
Configure Options¶
There are several options that control the build of EGL at configuration time
-D egl=enabled
By default, EGL is enabled. When disabled, the main library and the drivers will not be built.
-D platforms=...
List the platforms (window systems) to support. Its argument is a comma separated string such as
-D platforms=x11,wayland
. It decides the platforms a driver may support. The first listed platform is also used by the main library to decide the native platform.The available platforms are
x11
,wayland
,android
, andhaiku
. Theandroid
platform can either be built as a system component, part of AOSP, usingAndroid.mk
files, or cross-compiled using appropriate options. Unless for special needs, the build system should select the right platforms automatically.-D gles1=enabled
and-D gles2=enabled
These options enable OpenGL ES support in OpenGL. The result is one big internal library that supports multiple APIs.
-D shared-glapi=enabled
By default,
libGL
has its own copy oflibglapi
. This options makeslibGL
use the sharedlibglapi
. This is required if applications mix OpenGL and OpenGL ES.
Use EGL¶
Demos¶
There are demos for the client APIs supported by EGL. They can be found in mesa/demos repository.
Environment Variables¶
There are several environment variables that control the behavior of EGL at runtime
EGL_PLATFORM
This variable specifies the native platform. The valid values are the same as those for
-D platforms=...
. When the variable is not set, the main library uses the first platform listed in-D platforms=...
as the native platform.EGL_LOG_LEVEL
This changes the log level of the main library and the drivers. The valid values are:
debug
,info
,warning
, andfatal
.
Packaging¶
The ABI between the main library and its drivers are not stable. Nor is there a plan to stabilize it at the moment.
Developers¶
The sources of the main library and drivers can be found at
src/egl/
.
The code basically consists of two things:
An EGL API dispatcher. This directly routes all the
eglFooBar()
API calls into driver-specific functions.Two EGL drivers (
dri2
andhaiku
), implementing the API functions handling the platforms’ specifics.
Two of API functions are optional (eglQuerySurface()
and
eglSwapInterval()
); the former provides fallback for all the
platform-agnostic attributes (i.e. everything except EGL_WIDTH
& EGL_HEIGHT
), and the latter just silently pretends the API call
succeeded (as per EGL spec).
A driver _could_ implement all the other EGL API functions, but several of
them are only needed for extensions, like eglSwapBuffersWithDamageEXT()
.
See src/egl/main/egldriver.h
to see which driver hooks are only
required by extensions.
Bootstrapping¶
When the apps calls eglInitialize()
, the driver’s Initialize()
function is called. If the first driver initialization attempt fails,
a second one is tried using only software components (this can be forced
using the LIBGL_ALWAYS_SOFTWARE
environment variable). Typically,
this function takes care of setting up visual configs, creating EGL
devices, etc.
Teardown¶
When eglTerminate()
is called, the driver->Terminate()
function
is called. The driver should clean up after itself.
Subclassing¶
The internal libEGL data structures such as _EGLDisplay
,
_EGLContext
, _EGLSurface
, etc. should be considered base classes
from which drivers will derive subclasses.
EGL Drivers¶
egl_dri2
This driver supports several platforms:
android
,device
,drm
,surfaceless
,wayland
andx11
. It functions as a DRI driver loader. Forx11
support, it talks to the X server directly using (XCB-)DRI3 protocol when available, and falls back to DRI2 if necessary (can be forced withLIBGL_DRI3_DISABLE
).This driver can share DRI drivers with
libGL
.haiku
This driver supports only the Haiku platform. It is also much less feature-complete than
egl_dri2
, supporting only part of EGL 1.4 and none of the extensions beyond it.
Lifetime of Display Resources¶
Contexts and surfaces are examples of display resources. They might live longer than the display that creates them.
In EGL, when a display is terminated through eglTerminate
, all
display resources should be destroyed. Similarly, when a thread is
released through eglReleaseThread
, all current display resources
should be released. Another way to destroy or release resources is
through functions such as eglDestroySurface
or eglMakeCurrent
.
When a resource that is current to some thread is destroyed, the
resource should not be destroyed immediately. EGL requires the resource
to live until it is no longer current. A driver usually calls
eglIs<Resource>Bound
to check if a resource is bound (current) to
any thread in the destroy callbacks. If it is still bound, the resource
is not destroyed.
The main library will mark destroyed current resources as unlinked. In a
driver’s MakeCurrent
callback, eglIs<Resource>Linked
can then be
called to check if a newly released resource is linked to a display. If
it is not, the last reference to the resource is removed and the driver
should destroy the resource. But it should be careful here because
MakeCurrent
might be called with an uninitialized display.
This is the only mechanism provided by the main library to help manage the resources. The drivers are responsible to the correct behavior as defined by EGL.
EGL_RENDER_BUFFER
¶
In EGL, the color buffer a context should try to render to is decided by
the binding surface. It should try to render to the front buffer if the
binding surface has EGL_RENDER_BUFFER
set to EGL_SINGLE_BUFFER
;
If the same context is later bound to a surface with
EGL_RENDER_BUFFER
set to EGL_BACK_BUFFER
, the context should try
to render to the back buffer. However, the context is allowed to make
the final decision as to which color buffer it wants to or is able to
render to.
For pbuffer surfaces, the render buffer is always EGL_BACK_BUFFER
.
And for pixmap surfaces, the render buffer is always
EGL_SINGLE_BUFFER
. Unlike window surfaces, EGL spec requires their
EGL_RENDER_BUFFER
values to be honored. As a result, a driver should
never set EGL_PIXMAP_BIT
or EGL_PBUFFER_BIT
bits of a config if
the contexts created with the config won’t be able to honor the
EGL_RENDER_BUFFER
of pixmap or pbuffer surfaces.
It should also be noted that pixmap and pbuffer surfaces are assumed to
be single-buffered, in that eglSwapBuffers
has no effect on them. It
is desirable that a driver allocates a private color buffer for each
pbuffer surface created. If the window system the driver supports has
native pbuffers, or if the native pixmaps have more than one color
buffers, the driver should carefully attach the native color buffers to
the EGL surfaces, re-route them if required.
There is no defined behavior as to, for example, how glDrawBuffer
interacts with EGL_RENDER_BUFFER
. Right now, it is desired that the
draw buffer in a client API be fixed for pixmap and pbuffer surfaces.
Therefore, the driver is responsible to guarantee that the client API
renders to the specified render buffer for pixmap and pbuffer surfaces.
EGLDisplay
Mutex¶
The EGLDisplay
will be locked before calling any of the dispatch
functions (well, except for GetProcAddress which does not take an
EGLDisplay
). This guarantees that the same dispatch function will
not be called with the same display at the same time. If a driver has
access to an EGLDisplay
without going through the EGL APIs, the
driver should as well lock the display before using it.